Crystalline microstructures include micro-electromechanical systems (MEMS), monolithic integrated electronic and optoelectronic devices (e.g. optical ridge waveguides), solar cells, and other structures that are made from crystalline materials and are small in at least one dimension (e.g. thickness). Throughout the specification, MEMS are used as examples of crystalline microstructures and the definition of “MEMS” is expanded to include structures that do not necessarily include moving parts or electrical actuators, but that are made from crystalline bulk materials. Further, a crystalline microstructure or MEMS device (according to the expanded definition of “MEMS”) has at least one “small” dimension that accentuates the importance of surface properties in determining the structural characteristics of the device. For example, a solar cell may be many centimeters in length and width, but less than a millimeter in thickness. The small ratio of thickness to length and width means that surface properties of the solar cell are important determinants of its mechanical strength, and therefore that it is considered to be a “microstructure”. The most common crystalline microstructures, however, are MEMS devices in the traditional sense of that term.
Micro-electromechanical systems (MEMS) are mechanical devices roughly ranging in size from micrometers to millimeters, although the term “MEMS” is generalized somewhat in the specification. A subset of MEMS, albeit the majority of MEMS, is silicon MEMS, meaning devices that are fabricated from silicon wafers. Examples of silicon MEMS devices include accelerometers used to trigger airbag deployment in cars, and chips containing millions of microscopic mirrors that modulate light in projection displays.
Silicon MEMS began with the realization that silicon can be used as a mechanical material rather than (or in addition to) as an electronic material. The growth of silicon MEMS technology has been aided by the availability of silicon processing tools originally developed for the semiconductor integrated circuit industry. More recently, tools such as deep reactive ion etchers have been built specifically for MEMS fabrication.
Silicon wafers, the starting point for silicon MEMS device fabrication, are essentially perfect crystals in their bulk, but may have varying surface properties. A polished silicon surface has different properties than an etched surface, for example. In particular, different surfaces are characterized by different imperfections or deviations from the perfect bulk crystalline lattice. The surface properties of micro-scale devices are especially critical to performance because the surface to volume ratio is greater for small devices than for large ones.
Silicon and other crystalline materials are brittle. They can be stressed repeatedly to less than their breaking point with no ill effects. But once a silicon structure breaks, it is permanently destroyed. (In the specification “fracture” and “failure” are used interchangeably.) Structures made from brittle materials fail catastrophically when applied stress exceeds the strength of the weakest part of the structure. In silicon MEMS the maximum stress often occurs on a surface rather than in the bulk material and the weakest link is most often a surface flaw.
Currently, the reliability of silicon MEMS devices can be estimated from the results of destructive testing of finished devices. Simply put, one makes devices and then breaks them. This approach is inefficient because it leaves engineers without reliability data until late in the development cycle when a device has been fully designed and built. Furthermore, if problems are discovered during reliability testing, there is no assurance that whatever changes are made will have the desired effect. One must repeat the design-build-break cycle to see if modifications are successful. The duration and expense of this mode of development leads to designs that may be unnecessarily bulky as engineers try to avoid the risk of having to run extra development cycles.
What is needed is a method to predict failures in crystalline microstructures. Ideally, such a method would be sensitive to varying surface properties of crystalline materials, and offer predictive data before complex devices are fabricated.